(1) Field of the Invention
The present invention relates to a cell, and more specifically to a cell which secures the reliability of a protective circuit electrically connected with the cell.
(2) Description of the Related Art
Generally, a lithium ion cell is provided with an external protective circuit for controlling the cell voltage, thereby preventing overcharge and over-discharge. For example, a conventional lithium ion cell 11 shown in FIG. 1 is electrically connected with a protective circuit 13 via a lead plate 12, which can be made from nickel, nickel-plated stainless, or the like, considering its connectivity with the protective circuit 13.
On the other hand, an outer can 14 of the cell 11 whose voltage grows as high as 3.5-5.0V during a charging operation is not made of iron or stainless but of aluminum or an aluminum alloy. This is because unlike iron and stainless, aluminum does not cause the outer can 14 to corrode during a long-term preservation in a high voltage. In addition, unlike iron and stainless which have large specific gravity, aluminum which has small specific gravity allows the outer can 14 to have a large gravimetric energy density.
However, when it comes to the welding of the lead plate 12 made of nickel or the like and the outer can 14 made of aluminum or the like, the insolubility of the lead plate 12 makes it impossible to employ the ultrasonic welding and the good conductivity of the outer can 14 makes it impossible to employ the resistance welding. For this reason, the laser welding has been employed to combine the lead plate 12 and the outer can 14.
However, the laser welding also has a drawback that a laser beam causes the protective circuit 13 to be electrified, thereby deteriorating its reliability.